A module's composer.json file stipulates the filename where the Drush services (e.g. the Drush command files) are defined. The default services file is drush.services.yml, which is defined in the extra section of the composer.json file as follows:

"extra": {
"drush": {
"services": {
"drush.services.yml": "^9"
}
}
}

If for some reason you need to load different services for different versions of Drush, simply define multiple services files in the services section. The first one found will be used. For example:

In this example, the file drush-9-99.services.yml loads commandfile classes that require features only available in Drush 9.99 and later, and drush.services.yml loads an older commandfile implementation for earlier versions of Drush.

It is also possible to use version ranges to exactly specify which version of Drush the services file should be used with (e.g. "drush.services.yml": ">=9 <9.99").

In Drush 9, the default services file, drush.services.yml, will be used in instances where there is no services section in the Drush extras of the project's composer.json file. In Drush 10, however, the services section must exist, and must name the services file to be used. If a future Drush extension is written such that it only works with Drush 10 and later, then its entry would read "drush.services.yml": "^10", and Drush 9 would not load the extension's commands. It is all the same recommended that Drush 9 extensions explicitly declare their services file with an appropriate version constraint.

Drush command info (annotations) can be altered from other modules. This is done by creating and registering 'command info alterers'. Alterers are class services that are able to intercept and manipulate an existing command annotation.

In order to alter an existing command info, follow the next steps:

In the module that wants to alter a command info, add a service class that implements the \Consolidation\AnnotatedCommand\CommandInfoAltererInterface.

In the module drush.services.yml declare a service pointing to this class and tag the service with the drush.command_info_alterer tag.

In the class implement the alteration logic the alterCommandInfo() method.

Along with the alter code, it's strongly recommended to log a debug message explaining what exactly was altered. This would allow the easy debugging. Also it's a good practice to inject the the logger in the class constructor.

For an example, see the alterer class provided by the testing 'woot' module: tests/resources/modules/d8/woot/src/WootCommandInfoAlterer.php.

Commandfiles that don't ship inside Drupal modules are called 'global' commandfiles. See the examples/Commands folder for examples. In general, it's better to use modules to carry your Drush commands. If you still prefer using a global commandfiles, here are two examples of valid commandfile names and namespaces: